Abstract

Pulse amplitude modulated (PAM) fluorescence has been used as a proxy of microphytobenthic biomass after a dark adaptation period of 15 min to stabilise the minimum fluorescence yield (Fo15). This methodology was investigated for in situ migratory and ex situ engineered non-migratory biofilms, comparing dark adaptation to low (5% ambient) and far-red light treatments over different emersion periods. Far-red and low light reduced potential errors resulting from light history effects, by reversal of non-photochemical quenching after 5 min of treatment, compared to over 10 min required by conventional dark adaptation. An in situ decline of minimum fluorescence yield over 15 min was observed during the dark adaptation for migratory biofilms, but was not observed in the non-migratory biofilms indicating that the major cause of decline was downward vertical migration of cells into the sediment. This pattern occurred in far-red light after 10 min, but not for the low light treatment, indicating that low light maintained the biomass at the surface of the sediment. It is therefore concluded that low light treatment is a better option than conventional dark adaptation for the measurement of minimum fluorescence as a proxy of microphytobenthic biomass.

Abbreviations

Fo′, Fo5, Fo10, Fo15

Minimum fluorescence yield after 10 s and after 5, 10 and 15 min of treatment, respectively

Fm′, Fm5, Fm10, Fm15

Maximum fluorescence yield after a saturating pulse, after 10 s and after 5, 10 and 15 min of treatment, respectively

Fv′/Fm′, Fv5/Fm5, Fv10/Fm10, Fv15/Fm15

PSII photochemical efficiency after a saturating pulse, after 10 s or 5, 10 and 15 min of treatment, respectively

NPQ

Non-photochemical fluorescence quenching

PPFD

Photosynthetic photon flux density

PSII

Photosystem II

Communicated by M. Kühl, Helsingør

Notes

Acknowledgments

B. Jesus was funded by a PhD grant from FCT (Praxis XXI BD21634/99) and a Post Doctoral grant (POCI BPD/20993/2004). This work was also funded by the HIMOM project (Contract n° EVK3-2001-00043 I). The authors thank L. Ribeiro for the valuable help with diatom identification.